Device for controlling electric-motor power steering device

ABSTRACT

Distortions of motor voltage and current and a torque ripple of the motor are generated by dead band compensation for preventing an arm short circuit of an inverter, and a feeling of physical disorder is given in a handle operation. Therefore, the distortions of the motor voltage and current and the torque ripple of the motor are improved by using the dead band compensation for estimating the voltage distortion on the basis of a model current generated from a current command value such that no feeling of physical disorder is given in the handle operation of an electric power steering apparatus.

TECHNICAL FIELD

The present invention relates to a controller of an electric powersteering apparatus in which a steering assist force caused by a motor isgiven to a steering system of an automobile or a vehicle, andparticularly, relates to a controller of an electric power steeringapparatus for improving dead band control of an inverter for driving themotor.

BACKGROUND TECHNIQUE

In the electric power steering apparatus for energizing assist force byrotation force of the motor in a steering apparatus of the automobile orthe vehicle, the assist force is energized to a steering shaft or a rackshaft by a transmission mechanism of a gear or a belt, etc. through aspeed reduction gear with respect to driving force of the motor. Aninverter, etc. are used in a motor driving circuit to supply a currentto the motor so as to generate a predetermined desirable torque by thismotor.

Here, FIG. 1 shows a basic construction of an electric power steeringapparatus disclosed in a Japanese patent literature (JP-A-8-142884).FIG. 2 shows the details of a motor driving circuit within this electricpower steering apparatus. In FIG. 1, torque detected by a torque sensor103 is inputted to a phase compensator 121, and a torque command valueis calculated. Next, the torque command value is inputted to a currentcommand calculating element 122. A vehicle speed detected by a vehiclespeed sensor 112 is added and a current command value Iref is calculatedby the current command calculating element 122. In this control,feedback control is adopted. A current Imes of a motor 110 as a controlobject is detected by a motor current detecting circuit 142, and is fedback to a comparator 123. The current Imes is then compared with thecurrent command value Iref, and an error is calculated. So-calledproportion integration control of this error is performed by aproportion calculating element 125 and an integration calculatingelement 126. The current command value is inputted to a differentiatingcompensator 124 for improving a transient response. The respectiveoutputs of the differentiating compensator 124, the proportioncalculating element 125 and the integration calculating element 126 areadded by an adder 127, and a current control value E is calculated. Amotor driving circuit 141 supplies a current to the motor 110 on thebasis of the current control value E as an input value. A battery 114 isan electric power source of the motor driving circuit.

FIG. 2 shows details of the motor driving circuit 141. The motor drivingcircuit 141 is comprised of an inverter unit constructed by FET as aswitching element, and a gate control unit for controlling the operationof gate of FET. The inverter unit comprises an H-bridge which isconstructed by a up-and-down arm constructed by FET1 and FET3, or aup-and-down arm constructed by FET2 and FET4. In the gate control unit,the current control value E is inputted to a converting unit 130, and atiming signal with respect to each FET is generated and inputted to gatedriving circuits 133 a, 134 a, 133 b, 134 b. Thus, a gate signal able tooperate gate of FET is generated. However, the timing signal generatedby the converting unit 130 is not directly inputted to the gate drivingcircuits 134 a and 134 b, but is respectively inputted to a dead timecircuit 131 and a dead time circuit 132 because of the followingreasons.

Each up-and-down arm constituting the inverter unit, e.g., FET1 and FET3are alternately repeatedly turned on and off. Similarly, FET2 and FET4are alternately repeatedly turned on and off. However, the FET is not anideal switch. Therefore, no FET is turned on and off in a moment inaccordance with indication of the gate signal, but a turn-on time and aturn-off time are required. Therefore, when the indication of turning-onFET1 and the indication of turning-off FET3 are simultaneouslyperformed, a problem exists in that FET1 and FET3 are simultaneouslyturned on and the up-and-down arm is short-circuited. Therefore, when anoff-signal is given to the gate driving circuit 133 a so as not tosimultaneously turn-on FET1 and FET3, an on-signal is not immediatelygiven to the gate driving circuit 134 a, but is given to the gatedriving circuit 134 a by putting the pause of a predetermined timecalled a so-called dead time by the dead time circuit 131. Thus, theup-and-down short circuit of FET1 and FET3 is prevented. This alsosimilarly hold true in FET2 and FET4.

However, the existence of this dead time becomes a cause generating theproblem of torque insufficiency and a torque ripple in the control ofthe electric power steering apparatus. This problem will next beexplained in detail.

First, FIGS. 3A to 3D show the relation of the dead time, the turn-ontime and the turn-off time. In FIGS. 3A to 3D, a signal K is basicallyset to on and off signals with respect to FET1 and FET3. However, inreality, a gate signal K1 is given to FET1, and a gate signal K2 isgiven to FET2. Namely, the dead time Td is secured. A phase voltageconstructed by FET1 and FET2 is set to Van. Even when the on-signal dueto the gate signal K1 is given, the FET is not immediately turned on,but is turned on after a turn-on time Ton is required. On the otherhand, even when the off-signal is given, the FET is not immediatelyturned off, but is turned off after a turn-off time Toff is required.Vdc is an electric power voltage of the inverter.

Accordingly, a total delay time Ttot is expressed by the followingexpression 1.Ttot=Td+Ton−Toff   [Expression 1]

Here, the turn-on time Ton and the turn-off time Toff are changed bykinds, capacities, etc. of used FET and IGBT, etc. Further, the deadtime Td is generally a value greater than the turn-on time Ton and theturn-off time Toff.

Next, an influence affected by this dead time Td will be explained.

First, there is the following influence in the influence with respect tothe voltage. As shown in FIGS. 3A to 3D, the actual gate signals K1 andK2 with respect to an ideal gate signal K differ from the gate signal Kby the influence of the dead time Td. Therefore, distortion is generatedin the voltage. However, a value ΔV of this distortion voltage is shownin expression 2 when the direction of a motor current is positive (whenthe current is directionally flowed from the electric power source tothe motor). The value ΔV is shown in expression 3 when the direction ofthe current is negative (when the current is directionally flowed fromthe motor to the electric power source).−ΔV=−(Ttot/Ts)·(Vdc/2)  [Expression 2]

Where, Ts is an inverse number Ts=1/fs of a PWM frequency fs when theinverter is PWM-controlled.ΔV=(Ttot/Ts)·(Vdc/2)  [Expression 3]

When the above expressions 2 and 3 are represented by one expression,the following expression 4 is formed.ΔV=−sign(Is)·(Ttot/Ts)·(Vdc/2)  [Expression 4]

Here, sign(Is) represents the polarity of the motor current.

It is derived from the expression 4 that the influence of the dead timeTd with respect to the distortion voltage ΔV greatly appears as thefrequency fs is high and the electric power voltage Vdc is small.

The influence of the dead time Td with respect to the voltage distortionhas been explained. However, with respect to the current or torque,there is an unpreferable influence affected by the dead time Td. Withrespect to the current distortion, when the current is changed from thepositive current to the negative current or is changed from the negativecurrent to the positive current, a phenomenon (zero clamping phenomenon)for fixing the current to the vicinity of zero is generated by the deadtime Td. This is because there is a tendency intended to maintain thecurrent to be zero by a reduction in voltage due to the dead time Tdsince load (motor) is inductance.

Further, output deficiency of torque and an increase of the torqueripple appear as the influence of the dead time Td with respect totorque. Namely, the current distortion causes a higher harmonic wave ofa low order and this generation results in the increase of the torqueripple. Further, the output deficiency of torque is generated since thereal current influenced by the dead time Td becomes smaller than theideal current.

Various countermeasures, so-called dead band compensation has beenconsidered to prevent such an unpreferable influence of the dead timeTd. Its basic idea is to compensate the distortion voltage ΔVrepresented by expression 4. Accordingly, a correction is performed by acorrection voltage Δu represented by the following expression 5 tocompensate expression 4.Δu=sign(Is)·(Ttot/Ts)·(Vdc/2)  [Expression 5]

Here, it is a problem that polarity sign(Is) of the current Is can notbe correctly detected. When the polarity of the current Is is measured,it is difficult to correctly measure the polarity of the current Is bynoises of the PWM control and the above zero clamping phenomenon of thecurrent.

In many conventional dead band compensations (e.g. disclosed inliterature 1 (Ben-Brahim, The analysis and compensation of dead-timeeffects in the three phase PWM inverters, Proceedings of theIEEE-IECON98, Volume 2, pages 792-797)), methods are complicated and theaddition of hardware is required. Further, no countermeasure consideringa change of a load current such as the motor current, etc. is taken.

Therefore, the dead band compensation for preventing the up-and-down armshort circuit of the inverter causes distortions of the motor voltageand current or the output deficiency of torque and the increase of thetorque ripple. An improvement countermeasure of this dead bandcompensation is conventionally complicated, and the addition of hardwareis caused. Further, this compensation is imperfect dead bandcompensation in which no influence of the motor load current isconsidered.

The present invention is made from the above situations, and its objectis to provide a controller of an electric power steering apparatus inwhich distortions of motor voltage and current and torque ripple aresmall by using dead band compensation having a simple construction andalso considering influence of motor load current.

DISCLOSURE OF THE INVENTION

The present invention relates to a controller of an electric powersteering apparatus for controlling a current of a motor giving asteering assist force to a steering mechanism by using an inverter,based on a current command value calculated on the basis of at least onesteering torque signal generated in a steering shaft, and a voltagecommand value as an output of a current control circuit for setting atleast said current command value to an input. The above object of thepresent invention is achieved by a construction in which said controllercomprises a dead band compensating circuit in which a model current isgenerated based on said current command value, and a dead bandcompensation of said inverter is performed based on said model current.The above object of the present invention is also achieved by aconstruction in which an output value of said dead band compensatingcircuit is an adding value of a fixing value and a change value which isproportional to said model current.

Further, the above object of the present invention is achieved by aconstruction in which an output value of said dead band compensatingcircuit is a second change value proportional to said model current whensaid output value is a fixing value or less, and said output value is anadding value of said fixing value and a change value which isproportional to said model current when said output value is said fixingvalue or more. Further, the above object of the present invention isachieved by a construction in which said fixing value is a valuedetermined from the characteristics of switching element constitutingsaid inverter. Further, the above object of the present invention isachieved by a construction in which said model current is an outputvalue of a reference model circuit for setting said current commandvalue to an input value and constructed by a first order lag function.Further, the above object of the present invention is achieved by aconstruction in which a hysteresis characteristic circuit is arranged inthe input of said dead band compensating circuit. Further, the aboveobject of the present invention is achieved by a construction in which ahysteresis width of said hysteresis characteristic circuit is calculatedbased on the rotating speed of said motor or said current command value.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view showing control construction of an electric powersteering apparatus.

FIG. 2 is a view showing construction of a gate circuit considering adead time of an inverter of the electric power steering apparatus.

FIGS. 3A to 3D are views showing the relation of dead time, turn-on timeand turn-off time in switching of inverter.

FIG. 4 is a view showing a basic control system of the electric powersteering apparatus comprising dead band compensation according to thepresent invention.

FIG. 5 is a view showing a detailed construction within the dead bandcompensation of embodiment 1.

FIG. 6 is a view showing the relation of current command value Iref andactual motor current Imes.

FIG. 7 is a view showing characteristics of compensation value of thedead band compensation of embodiment 1.

FIG. 8 is a view showing a detailed construction within the dead bandcompensation as a modified example of embodiment 1.

FIG. 9 is a view showing characteristics of compensation value of thedead band compensation of the modified example of embodiment 1.

FIG. 10 is a view showing a detailed construction within the dead bandcompensation of embodiment 2.

FIG. 11 is a view showing characteristics of compensation value of thedead band compensation of embodiment 2.

FIG. 12 is a view showing a simulation result in which the presentinvention is applied to a sine wave motor.

FIG. 13 is a view showing a simulation result in which the presentinvention is applied to a rectangular wave motor.

FIG. 14 is a view showing the relation of rotating speed of the motor,current command value and hysteresis width.

FIG. 15 is a view showing characteristics of compensation value of thedead band compensation of embodiment 3.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention has a construction in which a model current Imodis generated from a current command value, and a polarity sign(Imod) ofcurrent and a amount of a distortion voltage ΔV are estimated on thebasis of the model current, and a dead band compensation is performed bycalculating a compensation value provided by adding polarity to theamount of the distortion voltage ΔV. An important feature of the presentinvention is that the actually measured motor current of which the abovepolarity is difficult to be correctly measured is not utilized byutilizing the model current.

Embodiment 1

Preferred embodiment 1 of the present invention will next be explainedin detail on the basis of the drawings.

FIG. 4 shows the basic construction of the controller of the electricpower steering apparatus. In a current command value calculating circuit4, a current command value Iref is calculated on the basis of a steeringtorque signal Tref generated in a steering shaft. On the other hand, acurrent Imes of a motor 1 is detected by a current detector 6, and isfed back to a subtracting circuit 7. The error between the above currentcommand value Iref and the motor current I is calculated and inputted toa current control circuit 3, and a voltage command value u iscalculated. Then, an inverter 2 is PWM-controlled on the basis of thevoltage command value u. The inverter 2 may be set to a single phaseinverter constructed two up-and-down arms as shown in FIG. 2, and may bealso set to a three-phase inverter constructed by three up-and-downarms.

A dead band compensating circuit 5 of the present invention is added tothe above basic control construction. Namely, in the dead bandcompensating circuit 5, the current command value Iref is set to aninput, and a compensation value Δu is calculated and added to thevoltage command value u as an output of the current control circuit 3 byan adding circuit 8.

Next, FIG. 5 shows the details of the dead band compensating circuit 5.First, the construction of the dead band compensating circuit 5 will beexplained, and its operation will be then explained.

The current command value Iref as the input of the dead bandcompensating circuit 5 is inputted to a reference model circuit 51, anda model current Imod is outputted. The first feature of the presentinvention is that the dead band compensation is performed by this modelcurrent Imod instead of the actually measured current of the motor.

A compensation value Δu is constructed by the polarity of thecompensation value Δu and the amount of the compensation value Δu(hereinafter noted as a compensation-value-amount Δu2).

First, the polarity of the compensation value Δu is calculated. Themodel current Imod as the output of the reference model circuit 51 isinputted to a polarity judging circuit 53, and its polarity is judged.Sign(Imod) as the output of the polarity judging circuit 53 is outputtedin the form of (+1) or (−1).

Next, the amount of the compensation value Δu, i.e., thecompensation-value-amount Δu2 is calculated. The model current Imodoutputted in the reference model circuit 51 is also used to calculatethe compensation-value-amount Δu2. First, the model current Imod isinputted to an absolute value circuit 55, and |Imod| as the output ofthe absolute value circuit 55 is inputted to a change value calculatingcircuit 56, and a change value Δu1 is calculated. The second feature ofthe present invention is that the dead band compensation consideringthis change value Δu1, i.e., the dead band compensation considering thechange of the motor current due to a motor load change is performed.

Then, a fixing value Δu0 set by a fixing value setting circuit 54 andthe change value Δu1 are added by an adding circuit 58, and its output(Δu0+Δu1) corresponds to the compensation-value-amount Δu2. Acompensation-value-amount calculating circuit corresponds to a portionsurrounded by a dotted line A of FIG. 5, and is constructed by theabsolute value circuit 55, the change value calculating circuit 56, thefixing value setting circuit 54 and the adding circuit 58. The modelcurrent Imod is inputted to the compensation-value-amount calculatingcircuit, and the compensation-value-amount is outputted as its output.

Finally, sign(Imod) as the output of the polarity judging circuit 53 andthe compensation-value-amount Δu2=(Δu0+Δu1) are inputted to amultiplying circuit 57 as one example of a polarity giving circuit, andthe compensation value Δu having the polarity is calculated as itsoutput. The basic construction of the dead band compensating circuit 5is provided as mentioned above. The operation of each circuit will nextbe explained in detail.

First, the reference model circuit 51 inputs the current command valueIref thereto and calculates the model current Imod. Here, a transferfunction of the reference model circuit 51 is represented by thefollowing expression 6.MR(s)=1/(1+Tc·s)  [Expression 6]

Here, Tc=1/(2π·fc) is set, and fc is a cutoff frequency of a currentcontrol loop.

This first order lag function is a model function of the current controlloop in which 1/(R+s·L) as a function representing the motor 1 of FIG. 4is derived on the basis of the current control circuit 3, the inverter 2and the current detecting circuit 6.

Here, one example of the relation of the current command value Iref andthe actual current Imes is shown in FIG. 6. The actual current Imesincludes many noises, which makes a polarity judgment difficult near azero current. Therefore, the motor current is generated through a firstorder lag circuit on the basis of the current command value Iref havingno noises without using the actual current Imes.

Next, the polarity of the model current Imod is inputted to the polarityjudging circuit 53, and sign(Imod) as the polarity of the model currentImod is calculated. As shown in expression 7, sign(Imod) has a value ofone of (+1) and (−1). As mentioned above, it is very difficult tomeasure the actual motor current and the inverter current and correctlyjudge the polarity since there are noises, etc. However, if the judgmentis made by using the model current as in the present invention, there isnot such a fear.Sign(Imod)=(+1) or (−1)  [Expression 7]

Next, the compensation value calculating circuit, i.e., a portion forcalculating the compensation-value-amount Δu2 as the amount of thecompensation value Δu from the model current Imod will be explained.First, the model current Imod is inputted to the absolute value circuit55, and |Imod| is outputted. The absolute value is calculated to firstunify the polarity since the amount of the compensation value iscalculated.

Next, the output |Imod| of the absolute value circuit 55 is inputted tothe change value calculating circuit 56, and a change value Δu1 iscalculated. Here, the relation of the input and output of the changevalue calculating circuit 56 is represented by following expression 8.Δu1=Req·|(Imod−Ic)|  [Expression 8]

Here, Req represents equivalent resistance. In this case, Imod>Ic isset. In the case of Ic>Imod>0, Δu1=0 is set. Namely, no compensation ofthe change value Δu1 is performed when the model current Imod is a smallvalue. In actual phenomenon, the change value hits the peak with respectto an increase of the current.

Here, it is important that the change value Δu1 represented byexpression 8 is changed in proportion to the model current Imod. Namely,an important point is that the change value Δu1 considering the changeof the motor current due to the change of a motor load is assembled intothe compensation value Δu. This point is not considered in theconventional dead band compensation.

On the other hand, a fixing value Δu0 is set in the fixing value settingcircuit 54. This fixing value Δu0 shows the value of the followingexpression 9.Δu0=(Ttot/Ts)·(Vdc/2)  [Expression 9]

Here, as shown in expression 1, the total lag time Ttot is set toTtot=Td+Ton−Toff. The dead time Td, the turn-on time Ton and theturn-off time Toff are values determined by the kind of a switchingelement used in the inverter, etc. For example, in the case of the FET,there are also characteristics in which the turn-on time Ton and theturn-off time Toff are increased as a rated voltage and a rated currentare increased. Namely, in the element of the FET used in the inverter oflarge capacity, the turn-on time Ton and the turn-off time Toff tend tobe increased. Further, when the turn-on time Ton and the turn-off timeToff are increased, the dead time Td is also increased so as not togenerate the short circuit of a up-and-down arm. Vdc is determined by abattery voltage.

As explained above, the fixing value Δu0 is a value determined by theinverter used in the electric power steering apparatus.

Next, the change value Δu1 and the fixing value Δu0 are added by theadding circuit 58 as shown in expression 10, and thecompensation-value-amount Δu2 is calculated.Δu2=(Δu0+Δu1)  [Expression 10]

The expression 10 means that the fixing value Δu0 determined by the kindof the inverter is set to a reference and the compensation-value-amountΔu2 is adjusted by the change value Δu1 provided by adding the influenceof the motor current to this fixing value Δu0.

Finally, the polarity is given to the compensation-value-amount Δu2, andthe compensation value Δu is calculated. Specifically, as shown in thefollowing expression 11, sign(Imod) as the output of the polarityjudging circuit 53 and (Δu0+Δu1) representing thecompensation-value-amount are multiplied by the multiplying circuit 57as one example of the polarity giving circuit.Δu=sign(Imod)·Δu2=sign(Imod)·(Δu0+Δu1)  [Expression 11]

Here, sign(Imod) is (+1) or (−1) as a value. Accordingly, thecompensation value Δu has a value of (Δu0+Δu1) or −(Δu0+Δu1). Thiscompensation value Δu is provided as shown in FIG. 7.

The compensation value Δu calculated in this way is added to the voltagecommand value u as the output of the current control circuit 3 shown inFIG. 4 by the adding circuit 8. The additional calculation of thecompensation value Δu with respect to the voltage command value u meansthat the compensation value Δu for improving voltage and currentdistortions and a torque ripple due to the dead time for preventing theshort circuit of the up-and-down arm is added to the basic control shownby the voltage command value u, and the control is performed.

If this embodiment is employed, the model current is used. Accordingly,it is possible to realize the dead band compensation for preventing thatthe distortions of the motor voltage and current are generated and thetorque ripple is increased by a simple control circuit constructionwithout using the polarity judgment of the actually measured currenthaving many noises and often judged in error.

FIG. 8 shows a modified example of embodiment 1. This modified exampleshows an embodiment in which a hysteresis circuit 52 is added to theoutput of the reference model circuit 51. In this modified example, whenthe load current passes through a zero point, it is prevented that thepolarity becomes unstable (chattering of the compensation value), andstable control can be performed. An improvement is performed withrespect to these points in this modified example. FIG. 9 shows thecompensation value Δu considering the hysteresis circuit 52. Sincenoises are few in the model current, the hysteresis width can be reducedin comparison with a case using the actual motor current. Accordingly,more accurate dead band compensation can be performed.

In this embodiment, the calculation is performed by separating thepolarity sign (Imod) and the amount Δu2 of the compensation value as aprocedure for calculating the compensation value Δu. However, the sameeffects are naturally obtained even when the compensation value Δu iscalculated without performing the separation.

Embodiment 2

In the embodiment 1 explained above, when the hysteresis circuit 52 isarranged and when no hysteresis circuit 52 is arranged, there is aportion suddenly changed from the fixing value −Δu0 to the fixing valueΔu0, or a portion suddenly changed from the fixing value Δu0 to thefixing value −Δu0 in the vicinity in which the model current Imod is0[A]. When the value of the fixing value Δu0 is small, this suddenchange does not give a feeling of physical disorder to the feeling of ahandle operation. However, when the value of the fixing value Δu0 islarge, a problem exists in that the feeling of the handle operationbecomes worse by this sudden change. It is considered that the value ofthe fixing value Δu0 becomes large in a case mainly using the FET oflarge capacity, etc. for a large-sized vehicle, etc.

Therefore, to solve this problem, characteristics of a second changevalue proportional to the model current Imod are provided instead of thefixing value Δu0 even in a small area (the fixing value Δu0 or less) ofthe model current Imod. The dead band compensation similar to that ofembodiment 1 is performed on the basis of (Δu0+Δu1) as the compensationvalue Δu provided by adding the change value Δu1 proportional to themodel current Imod and the fixing value Δu0 similarly to embodiment 1when reaching the fixing value Δu0.

FIG. 10 shows the details of the dead band compensating circuit 5 basedon this idea. FIG. 11 shows the characteristics of the compensationvalue of this dead band compensation. The dead band compensating circuitin FIG. 10 differs from the dead band compensating circuit of embodiment1 in a portion of the fixing value setting circuit 54. In embodiment 2,a second change value calculating circuit 60 for calculating a secondchange value Δu3 is arranged instead of the fixing value setting circuit54. The second change value calculating circuit 60 calculates the secondchange value Δu3 proportional to the model current Imod, and has aninsensitivity band in the model current Imod near 0[A]. The inclinationof a change of the second change value Δu3 and the magnitude of theinsensitivity band are determined by the kind of the FET, the feeling ofthe handle operation, etc. every specific device. The constructions andoperations of other portions of the dead band compensating circuit 5 arethe same as embodiment 1.

FIG. 11 shows dead band compensation characteristics generated by thedead band compensating circuit 5 of embodiment 2. In the features ofthese characteristics, chattering is prevented by the insensitivity bandof the second change value calculating circuit 60, and a sudden changefrom the fixing value Δu0 to the fixing value −Δu0 can be prevented bythe second change value Δu3. As its result, in embodiment 2, there is aneffect able to prevent the worsening of the feeling of the handleoperation which is a problem in embodiment 1.

FIGS. 12A, 12B and 13A, 13B show simulation results using the dead bandcompensation of the present invention.

FIG. 12A shows a case in which no dead band compensation of the presentinvention is applied to a three-phase sine wave motor. FIG. 12B showsthe result of the motor current of one phase amount when the dead bandcompensation is applied. As shown in FIG. 12A, when no dead bandcompensation is used, distortion is generated near a peak value of themotor current and a zero point. In contrast to this, when the dead bandcompensation of the present invention is performed as shown in FIG. 12B,no distortion of the current is almost seen in comparison with FIG. 12A.

FIG. 13A shows a case in which no dead band compensation is applied to athree-phase rectangular wave motor. FIG. 13B shows a result of the motorcurrent when the dead band compensation is applied. When no dead bandcompensation of FIG. 13A is performed, it is seen that the actualcurrent Imes is considerably distorted with respect to the currentcommand value Iref. When this distortion is seen in detail, theinfluence of the distortion near the zero current of a certain phaseaffects the distortion near a maximum current of another phase. In thesimulation of the sine wave current motor as shown in FIG. 12A,distortion is generated near zero and the maximum current in the currentwaveform, but the same reason as the reason of this generation happened.When the maximum current of the motor current is distorted, a torqueripple of the motor is greatly generated. On the other hand, when thedead band compensation of FIG. 13B is performed, the actual current Imesbecomes a value closer to the current command value Iref and the currentdistortion is clearly small. Namely, when the dead band compensation ofthe present invention is used, the distortion of the current is reducedand the torque ripple can be also restrained in the sine wave currentmotor and the rectangular wave current motor.

Embodiment 3

Next, an embodiment relating to an improvement of the hysteresis widthof a hysteresis characteristic circuit will be explained. A problemexists in that the model current Imod cannot be formed as a perfectmodel of the actual motor current Imes, and there is an error betweenthe model current Imod and the actual motor current Imes. This problemwill be explained with reference to FIG. 11.

FIG. 14A shows the relation of the model current Imod and the actualmotor current Imes when the current command value Iref is large and therotating speed ω of the motor is fast. On the other hand, FIG. 14B showsthe relation of the model current Imod and the actual motor current Imeswhen the current command value Iref is small and the rotating speed ω ofthe motor is slow.

When the polarity is judged by positiveness and negativeness of themodel current Imod at t=tA of FIG. 14A, the actual motor current Imesyet has the positive polarity and the dead band compensation of theincorrect polarity is performed although the polarity of the modelcurrent Imod is already changed from positiveness to negativeness. Thehysteresis is arranged to prevent this error. However, when the relationof the model current Imod and the actual motor current Imes of FIG. 14Aand the relation of the model current Imod and the actual motor currentImes of FIG. 14B are compared, the error is increased as the currentcommand value Iref is large and the rotating speed ω of the motor isfast. Namely, with respect to the hysteresis width, it is necessary todetermine the hysteresis width considering the current command valueIref or the rotating speed ω of the motor.

An embodiment for improving such a problem will be explained withreference to FIG. 15. The embodiment of FIG. 15 is an embodiment inwhich the model current Imod is used in only the polarity judgment. Thisembodiment is also an embodiment for calculating the hysteresis width byconsidering both the current command value Iref and the motor rotatingspeed ω. In its construction, the rotating speed ω of the motor isinputted to a hysteresis width calculating circuit 61, and a hysteresiswidth W1 as a reference is first determined. On the other hand, thecurrent command value Iref is inputted to the absolute value circuit 55,and the magnitude |Iref| of the current command value is calculated. Ahysteresis width W2 also considering the magnitude |Iref| of the currentcommand value Iref is then calculated as W2=|Iref|×W1 by a multiplier62. Polarity sign(Imod) is then calculated and outputted in a hysteresisadding polarity judging circuit 63 to which the calculated hysteresiswidth W2 is inputted.

Here, the operation of the hysteresis adding polarity judging circuit 63will be explained. When the model current Imod is greater than thehysteresis width W2, “+1” is calculated as the polarity sign(Imod).Conversely, when the model current Imod is smaller than the hysteresiswidth −W2, “−1” is calculated as the polarity sign(Imod). When the modelcurrent Imod is greater than the hysteresis width −W2 and is smallerthan the hysteresis width +W2, the previous polarity sign(Imod) is used.

Thus, the error in the detection of the polarity judgment due to anerror in the model formation of the model current Imod and the actualmotor current Imes can be prevented by determining the hysteresis widthW2 on the basis of the current command value Iref and the rotating speedω of the motor in the polarity judgment. In this embodiment, thehysteresis width W2 is calculated by using both the current commandvalue Iref and the rotating speed ω of the motor, but may be alsocalculated on the basis of one of the current command value Iref and therotating speed ω of the motor. Further, a table may be used when thehysteresis width W1 is determined from the rotating speed ω of themotor. A table may be also used to calculate only the absolute value|Iref| from the current command value Iref. In this embodiment, thecalculation of the absolute value is used as it is to reduce acalculating amount without passing through the table.

As explained above, the incorrect detection of the polarity judgment dueto the error in the model formation can be prevented by determining thehysteresis width for the polarity judgment on the basis of the currentcommand value or the rotating speed of the motor. Thus, effective deadband compensation can be performed.

As explained above, if the present invention is used, the model currentis used. Accordingly, it is not necessary to actually measure the motorcurrent difficult to judge the polarity. Further, the dead bandcompensation also considering a change of the motor current isperformed. Accordingly, it is possible to provide an electric powersteering apparatus small in the distortions of the motor voltage andcurrent and also small in the torque ripple while the short circuit ofthe up-and-down arm of the inverter is reliably prevented. Further, itis also an advantageous effect of the present invention that it is notnecessary to newly add hardware so as to apply the present invention.

In accordance with the controller of the electric power steeringapparatus of the present invention, the dead band compensation isperformed on the basis of the model current from the current commandvalue. Accordingly, differing from the conventional dead bandcompensation based on the actually measured current including noises, itis possible to provide a controller of the electric power steeringapparatus able to perform the dead band compensation small in thedistortions of the motor voltage and current or small in the torqueripple.

Further, the dead band compensation also considering the change of themotor current due to the change of a motor load is performed by usingthe model current. Accordingly, differing from the dead bandcompensation using only the fixing value as in the conventional case, itis possible to provide a controller of the electric power steeringapparatus able to perform the dead band compensation small in thedistortions of the motor voltage and current or small in the torqueripple.

INDUSTRIAL APPLICABILITY

In the controller of the electric power steering apparatus of thepresent invention, the dead band compensation is performed on the basisof the model current from the current command value. Accordingly,differing from the conventional dead band compensation based on theactually measured current including noises, it is possible to performthe dead band compensation small in the distortions of the motor voltageand current or small in the torque ripple.

1. A controller of an electric power steering apparatus for controllinga current of a motor giving a steering assist force to a steeringmechanism by using an inverter, based on a current command valuecalculated on the basis of at least one steering torque signal generatedin a steering shaft, and a voltage command value as an output of acurrent control circuit for setting at least said current command valueto an input, characterized in that: said controller comprises a deadband compensating circuit in which a model current is generated based onsaid current command value, and a dead band compensation of saidinverter is performed based on said model current.
 2. A controller of anelectric power steering apparatus according to claim 1, wherein anoutput value of said dead band compensating circuit is an adding valueof a fixing value and a change value which is proportional to said modelcurrent.
 3. A controller of an electric power steering apparatusaccording to claim 1, wherein an output value of said dead bandcompensating circuit is a second change value proportional to said modelcurrent when said output value is a fixing value or less, and saidoutput value is an adding value of said fixing value and a change valuewhich is proportional to said model current when said output value issaid fixing value or more.
 4. A controller of an electric power steeringapparatus according to claim 2 or 3, wherein said fixing value is avalue determined from the characteristics of switching elementconstituting said inverter.
 5. A controller of an electric powersteering apparatus according to any one of claims 1 to 3, wherein saidmodel current is an output value of a reference model circuit forsetting said current command value to an input value and constructed bya first order lag function.
 6. A controller of an electric powersteering apparatus according to claim 2, wherein a hysteresischaracteristic circuit is arranged in the input of said dead bandcompensating circuit.
 7. A controller of an electric power steeringapparatus according to claim 6, wherein a hysteresis width of saidhysteresis characteristic circuit is calculated based on the rotatingspeed of said motor or said current command value.
 8. A controller of anelectric power steering apparatus according to a claim 4, wherein saidmodel current is an output value of a reference model circuit forsetting said current command value to an input value and constructed bya first order lag function.